1. Field of the Invention
The present invention relates to a capacitor network of which line patterns are formed on both sides on a printed circuit board.
2. Description of the Related Art
A capacitor network typical in the related art comprises a plurality of component capacitors interconnected such that the capacitance (composite capacitance) of the capacitor network can be reduced. Each component capacitor is formed from a pair of metallic foil layers on opposite sides of a printed circuit board.
FIG. 11 shows the equivalent circuit of a typical capacitor network 500 as described above. The component capacitors C60 to C65 of this capacitor network 500 are formed from two metallic foil layers on opposite sides of a printed circuit board, and are connected in series. The capacitance of this capacitor network 500 is the sum of the capacitance of each component capacitor C60 to C65. As a result, the capacitance of the capacitor network 500 can be reduced by an amount equal to the capacitance of the last capacitor C65 in this example by cutting the lines at points P50 and P51 to isolate the capacitor C65 from the network. The capacitance of the capacitor network 500 can be further reduced by an amount equal to the capacitance of capacitor C64, for example, by similarly cutting the lines at points P52 and P53.
It will be clear that the capacitance of this capacitor network 500 can only be adjusted downward by electrically isolating a number of component capacitors from the network. This creates the problem of not being able to optimally adjust the capacitance of the capacitor network 500 in certain cases.
Consider, for example, a contactless IC card in which an LC resonance circuit containing a capacitor network 500 as a circuit element is used as an antenna, and the capacitance of the capacitor network 500 is set to the capacitance (the "optimum capacitance" below) providing the greatest read/write distance to the reader/writer. The optimum capacitance is determined in this case by actually adjusting (reducing) the capacitance of the capacitor network 500 while measuring the read/write distance to the reader/writer.
The value of this optimum capacitance is the capacitance at which the resonance frequency of the LC resonance circuit of the contactless IC card matches the operating frequency of the reader/writer. With the above-noted method of setting the optimum capacitance, however, it is not possible to predict at what point the capacitance of the capacitor network 500 will be closest to the optimum capacitance. As described above, the capacitance of the capacitor network 500 can only be reduced in increments equal to the capacitance of the component capacitors. The capacitance of the capacitor network 500 is therefore always set to the last-adjusted value, that is, to the capacitance at which the network capacitance is less than the optimum capacitance or the capacitance resulting in a short circuiter read/write distance. It should be noted that while it is possible to increase the capacitance of the capacitor network 500 in such cases, this requires reconnecting the cut lines, a complicated and costly task.
For some applications a variable capacitor component can be mounted on the circuit board in place of a capacitor network 500, but these components are typically thick, large, and expensive. Such capacitor components also cannot be used on certain products having specific thickness limitations, including credit-card-size contactless IC cards with an ISO-standard 0.76 mm thickness limit.
It is noted that Japan Unexamined Patent Publication (kokai) S62-233913 (1987-233913) teaches a capacitor whereby the capacitance distributed to the front and back of the circuit board through through-holes is controlled using the circuit board thickness. The problem with this design, however, is that once the thickness of the circuit board is fixed, the capacitance of the capacitor cannot be changed. The above-described problem is therefore not resolved.
There is therefore a need for a capacitor network of simple configuration whereby the composite capacitance of the capacitor network can be easily increased and decreased.